As the process for industrial production of aminocarboxylic acid salts, there is currently used generally the Strecker process in which a glycine salt, an iminodiacetic acid salt, a nitrilotriacetic acid salt or the like is obtained using hydrocyanic acid and formaldehyde as raw materials. However, since hydrocyanic acid is a virulently poisonous gas, the Strecker process has big limitations in production facility, handling, production site, etc. Moreover, since hydrocyanic acid is mostly obtained as a by-product in acrylonitrile production, the Strecker process has also had a big problem in stable procurement of the raw material.
There are also known processes in which an aminoalcohol is subjected to oxidative dehydrogenation in a caustic alkali to produce an aminocarboxylic acid salt (U.S. Pat. No. 2,384,816, U.S. Pat. No. 2,384,817, U.S. Pat. No. 3,535,373, U.S. Pat. No. 3,842,081, U.S. Pat. No. 3,739,021, etc.). U.S. Pat. No. 2,384,816 discloses a process in which an aminoalcohol and an alkali metal hydroxide are reacted using no catalyst. The process, however, requires a long reaction time and moreover produces an aminocarboxylic acid salt at a low yield. U.S. Pat. No. 2,384,817 discloses a process in which monoethanolamine and potassium hydroxide are reacted in the presence of a copper catalyst and in the absence of water to obtain potassium glycinate. According to the finding by the present inventors, the process produces said glycinate at an insufficient yield. U.S. Pat. No. 3,578,709 discloses a process in which triethanolamine and an alkali hydroxide are reacted in the presence of a zinc oxide catalyst to obtain a nitrilotriacetic acid salt. The process is not satisfactory in the yield of the nitrilotriacetic acid salt. U.S. Pat. No. 3,842,081 discloses the reaction of diethanolamine and potassium hydroxide in the presence of cadmium oxide to obtain potassium iminodiacetate at a relatively high yield. U.S. Pat. No. 3,535,373, U.S. Pat. No. 3,578,709 and U.S. Pat. No. 3,739,021 disclose the reaction of triethanolamine and an alkali hydroxide in the presence of cadmium oxide to obtain a nitrilotriacetic acid salt at a relatively high yield. In these processes using cadmium oxide as a catalyst, however, there is a fear that the reaction products are contaminated with toxic cadmium compounds; accordingly, the products have no utilizability depending upon the applications and moreover there is also a problem of disposing wastes. Therefore, these processes have been unable to become a technique competitive with the Strecker process.
Also, there are known processes in which an aminoalcohol is reacted in the coexistence of an alkali hydroxide, water and a copper-containing catalyst or a copper-zirconium-containing catalyst to obtain an aminocarboxylic acid salt (U.S. Pat. No. 4,782,183). In these processes, however, although the selectivity of desired aminocarboxylic acid salt is high (95 %), the repeated use of catalyst tends to result in a reduced selectivity and increased amounts of by-products. The main by-products are an oxalic acid salt when a glycine salt is produced from monoethanolamine as a raw material, a glycine salt when an iminodiacetic acid salt is produced from diethanolamine as a raw material, and an iminodiacetic acid salt, a glycine salt, etc. when a nitrilotriacetic acid salt is produced from triethanolamine as a raw material. Since these by-products have reactivities similar to those of desired aminocarboxylic acid salt, when the aminocarboxylic acid salt is used as a raw material for agricultural chemicals, drugs, etc., the yield of final product is affected greatly. Therefore, in order to obtain an aminocarboxylic acid salt of high purity, it is necessary to change the catalyst used, in a short period to maintain a high selectivity or to subject the obtained aminocarboxylic acid salt to a complex purification step.
The object of the present invention is to provide a novel process for producing an aminocarboxylic acid salt, which has no toxicity problem, which generates only small amounts of by-products, which gives a high yield and a high selectivity, and accordingly which can produce an aminocarboxylic acid salt economically and advantageously.